Welcome. If you decide to not evolve Wartortle, you will notice that it will be significantly weaker late-game compared to Blastiose, but the gap in base stats isn't that big, so you might be able to get away with it.
Here's an idea:
Make the bicycle shop sell bike paint. It's a relatively straight forward money sink, with the paint being intermediately expensive and wears out after a certain amount of steps/in-game hours